I. Field of the Invention
This invention relates to a heat treatment process for aluminum alloy sheet material that improves the paint bake response of the material.
II. Description of the Prior Art
Aluminum alloy sheet is being used more extensively nowadays as a structural and closure sheet material for vehicle bodies as automobile manufacturers strive for improved fuel economy by reducing vehicle weight. Traditionally, aluminum alloy is either direct chill cast as ingots or continuous cast in the form of a thick strip material, and then hot rolled to a preliminary thickness. In a separate operation, the strip is then cold rolled to the final thickness and wound into coil. The coil must then undergo solution heat treatment to allow strengthening of the formed panel during paint cure.
Solution heat treatment involves heating the metal to a suitably high temperature (e.g. 480.degree.-580.degree. C.) to cause dissolution in solid solution of all of the soluble alloying constituents that precipitated from the parent metal during hot and cold rolling, and rapid quenching to ambient temperature to create a solid supersaturated solution (see, for example, "Metallurgy for the Non-Metallurgist", published in 1987 by the American Society for Metals, pp 12-5, 12-6). Then the metal is precipitation hardened by holding the metal at room temperature (or sometimes at a higher temperature to accelerate the effect) for a period of time to cause the spontaneous formation of fine precipitates. The metal may then additionally undergo cleaning, pretreatment and prepriming operations before being supplied to a vehicle manufacturer for fabrication into body panels and the like.
It is highly desirable that the alloy sheet, when delivered to the manufacturer, be relatively easily deformable so that it can be stamped or formed into the required shapes without difficulty and without excessive springback. However, it is also desirable that the sheets, once formed and subjected to the normal painting and baking procedure, be relatively hard so that thin sheet can be employed and still provide good dent resistance. The condition in which the alloy sheet is delivered to the manufacturer is referred to as T4 temper and the final condition of the alloy sheet after the paint/bake cycle (which can be simulated by a 2% stretch and baking at 177.degree. C. for 30 minutes) is referred to as T8X temper. The objective is therefore to produce alloy sheet that has relatively low yield strength in T4 temper and high yield strength in T8X temper.
A drawback of the conventional solution heat treatment followed by the conventional age hardening procedure is that the so-called "paint bake response" (the change in yield strength from a desirable T4 temper to a desirable T8X temper caused by painting and baking) may suffer.
Another drawback of certain prior art solution heat treatment processes is that they require the metal to be treated in coiled form and, as a result (because of the large bulk of metal that has to be treated at one time), in a batch operation where heat treatment conditions are less controlled, holding times are longer, precise and uniform temperature control is difficult to obtain and high heating and cooling rates cannot be achieved.
There is therefore a need for improved treatments of aluminum alloy sheet material that can enhance the paint bake response (the T4 to T8X strength increase) and that preferably can be carried out continuously, i.e. on a section of the moving sheet as the sheet is processed in a coil to coil treatment line.
Japanese patent publication JP 5-44,000, assigned to Mitsubishi Aluminum KK and published on Feb. 23, 1993 discloses a reversion treatment for aluminum sheet whereby the T4 yield strength is lowered (for better formability) after a long period of natural age hardening. Following, a solution heat treatment, quench and natural age hardening, the aluminum sheet is heated to 200.degree.-260.degree. C. and held at the peak metal temperature for 3-80 seconds.
Japanese patent publication JP 5-279,822 assigned to Sumitomo Light Metal Industries Co. and published in Oct. 28, 1993 discloses a heat treatment of aluminum alloy to improve the paint bake response. Following solution heat treatment and quenching, the aluminum alloy sheet is heated to 15.degree.-120.degree. C. within 1 day for one hour or less, and is then further heated to 200.degree.-300.degree. C. for one minute or less.
Japanese patent publication JP 2-209,457 assigned to Kobe Steel Ltd. and published on Aug. 20, 1990 discloses a modification to a conventional continuous anneal solution heat treatment line to improve the paint bake response of aluminum sheet material. A reheating device is added to the end of the line to reheat the aluminum sheet immediately following solution heat treatment and quenching.
These references do not, however, result in the desired degree of improvements.